Field of the Invention
This invention relates to a pliable heating device having a flexible electrical heating apparatus, which is operated by a control device and which has at least one flexible heating element connected to a flexible support and that has a heating conductor situated in a heating circuit, and a flexible sensor conductor separated from the heating conductor by an intermediate insulation, having a dampable oscillator, which is contained in the control device and is connected to the sensor conductor and whose output signal can be varied as a function of various functional states of the heating apparatus. The functional states can be detected by the sensor conductor, and there is an evaluation device by which fault states can be detected from the output signal.
Discussion of Related Art
A pliable heating device is disclosed in German Patent Reference DE 10 2008 006 017 B4. In this known heating device, as in electric blankets, heating pads, and heated mattress covers, a heating element preferably embodied in the form of a heating cord is embedded in a flexible support. It has a heating conductor, a sensor conductor, an interposed electrically insulating intermediate layer, and an outer insulation layer. The heating conductor and the sensor conductor are connected to a control device that can control or regulate the heating operation and also can monitor the proper function of the heating device. In order to control or regulate the heating output, the control device has at least one circuit breaker situated in the same heating circuit as the heating conductor and can be controlled as a function of the desired heating output while complying with safety criteria. In order to detect functional states, in particular fault states, of the heating device, the sensor conductor is connected to a dampable oscillator circuit whose output signal is processed with regard to the functional states in an evaluation device. The output signal of the oscillator in this case also changes in particular as a function of different functional states of the intermediate insulation between the sensor conductor and the heating conductor, which insulation forms an essential sensor element and advantageously has a negative thermal response of its resistance value (NTC behavior), the temperature dependence is preferably exponential. The intermediate insulation can be used, for example, to ascertain locations with excessive heating of the heating element, so-called hot spots. The intermediate insulation can be embodied as fusing, such as irreversible and irreparable, thus making it possible to reliably detect mainly short circuits. It can also be embodied, for the usual temperatures that are to be detected, as non-fusing. If only the resistance change and not fusing is used in order to switch off the heating device, then a faulty functional state can be reversibly remedied. Other evaluation options and control options are achieved if the sensor conductor and/or the heating conductor has temperature-dependent resistance behavior, in particular a positive thermal response of the resistance value (PTC behavior). With this monitoring system that includes the oscillator circuit, it is possible to ascertain and differentiate among a variety of functional states. Depending on the functional state, the output signal of the oscillator can experience different signal changes, such as an amplitude change, a phase change, or a change in a pulse-pause ratio, which can be evaluated by the evaluation device individually or in various combinations in order to determine the functional state of the heating device. Pliable heating devices, however, are subject to a variety of influences such as aging, frequent washing, improper use, component tolerances, and the like, which also result in the signal changes of the oscillator and can lead to difficulties in evaluating states of the heating device and lack of reliability of the evaluations carried out and to associated incorrect interpretations.
In another pliable heating device disclosed by PCT Patent Reference WO 2005/118202 A2, the control device is embodied in a particular way to produce and evaluate a phase shift. In this device as well, there can be cases in which a fault recognition and evaluation are difficult.
U.S. Patent Application Publication 2011/259872 A1 discloses another pliable heating device in which the heating element is also embodied in the form of a heating cord that has a heating conductor, a sensor conductor, and an NTC intermediate insulation in order, for example, to be able to detect fault states and if necessary, to switch off the heating device. Here, also, the heating conductor and the sensor conductor can have a positive thermal response of their resistance value (PTC behavior) in order to thus control the heating output.